There is a technique for protecting content by giving a key for reproducing the content only to an authorized reproduction device. However, when the key for reproducing the content is leaked, even an unauthorized device can reproduce the content. In order to prevent damages caused by the leaked key, it is necessary to invalidate the leaked key.
As a technique for invalidating a leaked key, there is a copyright protection technology by media key block (MKB). By using this technology, when there is a content reproduction device that has been duplicated in an unauthorized manner using a leaked key, it is possible to invalidate the leaked key by identifying a device key used in the duplicated device and then appropriately updating MKB. In a typical method for identifying a leaked device key, a duplicated reproduction device is obtained, and analysis is performed off-line to identify a device key used in the duplicated reproduction device.
However, in the conventional technique, when a duplicated device is difficult to be obtained such as a case where a duplicated device is not commercially available, it has been difficult to identify a leaked device key.